Description

Gap junctions are clusters of intercellular channels connecting adjacent cells and permitting the direct exchange of ions and small molecules between cells. These channels are composed of two hemichannels, or connexons, one located on each of the two neighboring cells. Each connexon is composed of 6 trans-membrane protein subunits of the connexin (Cx) family. A gap of approximately 3 nm remains between the adjacent cell membranes, but two connexons interact and dock head-to-head in the extra-cellular space forming a tightly sealed, double-membrane intercellular channel (see Segretain and Falk, 2004). The activity of these intercellular channels is regulated, particularly by intramolecular modifications such as phosphorylation which appears to regulate connexin turnover, gap junction assembly and the opening and closure (gating) of gap junction channels.
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One mechanism of transport of connexon-containing vesicles involves movement along microtubules (Segretain and Falk, 2004). Such a transport system has been described for similar secretory vesicles (Toomre et al., 1999). Direct microtubule-dependent transport of connexons to GJ-assembly sites has recently been reported as well (Shaw et al., 2007).

Connexin-interacting proteins appear to function in regulating gap junction formation and communication. ZO-1 has been shown to alter the membrane localization of Cx43 and plays a role in regulating Cx43-mediated gap junctional communication in osteoblastic cells (Laing et al. 2005). ZO-1 may function in the delivery of Cx43 from a lipid raft domain to gap junctional plaques, which may be an important regulatory step in gap junction formation.

Connexins (Cxs) are encoded by a large gene family predicted to include at least 20 isoforms in humans. Most mammalian Cx genes consist of two exons. The first consists of untranslated sequence, and the second contains the entire coding sequence. Exceptionally, Cx36 and Cx45 contain 3 exons and 2 introns and the third exon contains the coding sequence (Belluardo et al. 1999 ; Jacob and Beyer 2001). Connexins have been divided in two major subgroups, alpha and beta, according to their amino acid sequence similarity (see Bruzzone et al., 2001; Willecke et al., 2002). Alternative names and additional subgroups have been suggested as well. Cx are synthesized by ribosomes in the endoplasmic reticulum (ER) membrane. All Cx proteins contain four trans-membrane domains (TM1 to TM4), two extracellular loops (E1 and E2) and one cytoplasmic loop. The amino- and carboxyl termini are located in the cytosol (reviewed in Segretain and Falk, 2004). After targeting to the ER, connexins are checked by a quality control system to prevent misfolded forms from progressing through the secretory pathway. Aberrant proteins are removed by endoplasmic-reticulum-associated degradation (ERAD).

Oligomerization of connexins Cx32 and Cx26 has also been observed in the ER-Golgi-intermediate compartment (ERGIC) (Diez et al. 1999). Heteromeric connexons containing both Cx32 and Cx26 have been observed. For the sake of simplicity, the connexon here is described as containing equal numbers of Cx26 and Cx32 subunits, although the ratio may vary.

Junctional channels are an assembly of two docked connexons on adjacent cells that permits direct communication of the cytoplasm in the two cells as shown below. Proteins associated with GJs such as catenins (Wu et al., 2003, Shaw et al., 2007) and L-CAM (Musil et al., 1990) might be required for connexon docking. Docking occurs through a tight interaction of the extracellular loops (Unger et al., 1999; Sosinsky and Nicholson, 2005). Intramolecular disulfide bridges between the two extracellular loops (E1 and E2) of connexin polypeptides are important for the correct three-dimensional structure of the extracellular loops (Foote et al., 1998)

Once transported to the plasma membrane, junctional channels aggregate into clusters forming gap junction plaques that may contain a few to many thousands of individual channels and that vary in size from a few square nanometers to many square micrometers (Bruzzone et al. 1996; Falk 2000; Severs et al. 2001). Gap junction plaques are involved in numerous processes including growth and differentiation (Loewenstein and Rose 1992), pathological cell proliferation (Roger et al. 2004; Segretain et al. 2003) and spermatogenesis (Juneja et al. 1999; Plum et al. 2000). The physiological importance of gap junction plaques is underscored by the diverse pathologies associated with connexin gene mutations (De Maio et al. 2002). An arbitrary number (10) of channels is shown as aggregating in this reaction but the actual number may be hundreds to thousands.

Docking of Cx43 at the plasma membrane may involve ZO-1 as well as alpha- and beta-catenin (Shaw et al., 2007). The role of ZO-1 in regulating gap junction biology is unclear. Recent results indicate a role for ZO-1 in regulating gap junction plaque size (Hunter et al., 2007).

Dab2 is recruited to Cx43-based GJs possibly through a direct interaction between its N-terminal phosphotyrosine binding (PTB) domain and a putative XPXY internalization motif found in the C-terminal tail of Cx43 as well as a number of other connexin family members (Piehl et al., 2007).The distal portion of Dab2 on its opposite end binds the globular N-terminal domain of clathrin heavy chains (Piehl et al., 2007).

Connexons may also traffic using a microtubule-independent mechanism. A few studies suggest that rough ER membranes can directly transfer connexons to the plasma membrane (Martin et al. 2001; Bloom and Goldstein 1998). Other cytoskeletal components, such as actin filaments, might be involved in the delivery of connexons to gap junction plaques (Thomas et al. 2001; Gilleron et al. 2006).